Gasification of the biomass consists in breaking down in the presence of a reactive gas (oxygen for example) a solid, for example wood, in order to obtain a gaseous product. During this process, the biomass is subjected to the following four successive thermochemical phenomena: drying, pyrolysis, oxidation and reduction.
Drying: the humidity of the fuel is removed via evaporation. This operation is endothermic, it takes place at a temperature typically between 100° C. and 160° C.
Pyrolysis: combustible and non-combustible gases are released by the dry biomass starting at 250° C. These gases are comprised of non-condensable vapors (methane, hydrogen, carbon monoxide, carbon dioxide, etc.) and of condensable vapors (tars). The residue of this operation called coke is carbon that contains mineral matter.
Oxidation: this takes place in the presence of the reactive gas (air, water vapor, pure oxygen, hydrogen) which conditions the calorific value of the gas at the outlet of the gasifier. The optimizing of the oxidation area is essential in that a strong proportion of tars produced during pyrolysis is cracked therein. The use of air, as a reactive gas, is the most common. In this specific case, the oxidation or partial combustion is the phase that provides the heat required for the three phases of the gasification process.
Reduction: the coke reacts with the water vapor and the carbon dioxide, forming hydrogen and carbon monoxide, the main constituents of the combustible gas produced.
Different technologies have been developed in order to implement the gasification of the biomass on an industrial scale. The most widespread technologies are the fixed bed gasifiers and the mobile bed gasifiers. The latter are intended for high thermal power installations (greater than 10 MW) and require a finely ground fuel.
Fixed bed gasifiers are intended for lower power installations and can use rougher fuel (for example wood chips). Two major categories of fixed bed gasifiers can be distinguished by the relative direction of circulation of the biomass and of the air: counter-current or co-current. In a counter-current gasifier, the supply with biomass is carried out via the top of the reactor and the air is injected by the bottom of the unit through a grid. The coke undergoes partial combustion which provides the thermal energy required for the various steps of the process. The gas passes through the reduction and pyrolysis areas and cools down by drying the biomass. This type of gasifier produces a lot of tars that must be eliminated at the output of the gasifier in order to be able to use the synthesis gas produced.
In a co-current gasifier, the supply with biomass and with oxidizing agent is carried out in the same direction. The gas produced passes through the hot zone which makes it possible to crack the tars formed during the pyrolysis reaction. As a consequence the gas produced leaves the reactor at a high temperature, of a magnitude of 700° C. The tar content is therefore much less than in the case of the counter-current gasifier. On the other hand, the existing co-current gasifiers are limited in terms of maximum power, due to the fact that the injecting of oxidizing agent (air, oxygen, water vapor) is carried out at the periphery, which limits the penetration of the reactive gas (also called “gasification agent” here) in the bed, in particular on the reduction area. Such a gasifier is described for example in patent application WO 2009/020442 (Detes Maden Enerji Ve Cevre Teknoloji Sistemleri Limited Sirket).
Solutions have been proposed to resolve this problem. U.S. Pat. No. 594,540 (Kitson, 1897) describes a co-current cylindrical gasifier wherein the entry of the air is carried out via the top of the cylinder. The air inlet nozzle has the shape of a cone, and the air is conveyed directly into the bed of biomass.
Likewise, U.S. Pat. No. 4,306,506 (Energy Recovery Research Group) describes a co-current cylindrical gasifier comprising successively from top to bottom an upper drying area, then a distillation area (pyrolysis), then an oxidation area, and finally a reduction area. The air is introduced via a duct that opens into the “core” of the oxidation area and comprising a cone for deflecting that sends air to the top and towards the bottom of all of the oxidation area. Furthermore, air can also be sent into the bottom of the reduction area, with this air being above all intended to cool the reduction area. In this configuration, injecting air results in two disadvantages: on the one hand, the oxygen of the air reacts with the hydrogen of the synthesis gas produced, which decreases the calorific power of the synthesis gas, and on the other hand, the synthesis gas is diluted by the nitrogen present in the air.
U.S. Pat. No. 4,568,271 (Kernforschungsanlage Jülich) describes a gasifier for the gasification of liquid effluents that contain organic compounds. The liquids are introduced into a vertical cylindrical container via a duct located in the top of the gasifier. An incandescent bed containing materials rich in carbon is located in the bottom of the cylindrical container. Oxygen is introduced into the incandescent bed via a central duct placed in the bottom of the cylindrical container then by a cone comprising openings and located in the incandescent bed. The liquid effluents are vaporized and “cracked”. This device is suitable only for treating liquid effluents.
German Patent Application DE 102010033646 (Pyrox GMBH) describes a co-current fixed bed gasifier comprising a “separated” oxidation chamber placed inside the body of the gasifier. This oxidation chamber comprises a tapered upper area with the wide portion of the cone located at the bottom, an oxidizing agent (air) inlet is provided in the upper portion of the oxidation chamber. The tapered upper area further comprises a double wall inside of which the pyrolysis gases produced in the pyrolysis area are conveyed, and introduced into the oxidation chamber. These pyrolysis gases pass through a grid located on the periphery of the oxidation chamber. The oxidation chamber described in German Patent application DE 102010033646 typically has a burner arrangement. This device is of complex construction, in addition the pyrolysis gases are highly loaded with tars, and the inlet grid risks becoming clogged quickly.
Patent application NL 8200417 (TAB BV) describes a co-current fixed bed gasifier able to treat a large variety of fuels. This gasifier comprises an air inlet in the top of the body of the gasifier, with this air being brought by a pipe in a chamber comprising a tapered upper portion with the wide portion of the cone located at the bottom. Due to the shape of the chamber, the diffusing of the air in the biomass is not optimal.
The co-current fixed bed gasifiers according to prior art are therefore limited in power. There exists a need for a co-current fixed bed biomass gasification device that makes it possible to lift the limitations of prior art in terms of maximum power, able to operate in particular at a maximum power greater than 500 kW, and which makes it possible to obtain a synthesis gas with a high output, minimum tar content, and a minimum carbon in the ash.